Inkjet printing is generally accomplished by expelling droplets of ink from tiny orifices to land on a recording medium such as paper. The most widespread technologies used to expel ink are a thermal process in which ink is vaporized to provide momentum to the ink droplet and a piezoelectric process in which an electromechanical force expels the ink droplet. Although the present invention is applicable to at least these two technologies, the preferred embodiment will be explained using the thermal inkjet process.
A typical inkjet cartridge is shown in FIG. 1, in which the cartridge body member 101 houses a supply of ink and routes the ink to the printhead 103 via ink conduits. Visible at the outer surface of the printhead are a plurality of orifices 105 through which ink is selectively expelled upon commands of the printer (not shown), which commands are communicated to the printhead through electrical connections 107. In one implementation of an inkjet print cartridge, the printhead 103, the orifices 105, and the electrical connections 107 are realized using a technology commonly known as Tape Automated Bonding (TAB). A flexible polymer tape 109, which may be formed of Kapton.TM., commercially available from 3M Corporation, or similar material which may be photoablated or chemically etched to produce the orifices 105 and other desirable characteristics is formed to have the plurality of orifices extending through the tape. Further copper or other conductive traces are deposited or created on one side of the tape so that electrical interconnections can be made from the printer to a semiconductor substrate. The semiconductor substrate includes thin film resistors for vaporizing the ink and may also contain transistors for multiplexing the electrical signals from the printer to the heater resistors. The semiconductor substrate is typically affixed to the tape by a photo definable adhesive barrier layer which is secured by a process of heat and pressure. Subsequently the tape 109 is typically bent around edges of the print cartridge and secured with an adhesive to the body to provide an uncluttered working surface for the printhead and to enable connection to the printer for the electrical contacts 107.
A close up view of one conventional orifice and firing chamber is illustrated in FIG. 2. The tape 109 is arranged such that the orifice 105, which extends from the inner surface of the tape to the outer surface, is centered essentially over a heater resistor 201 in an ink ejection chamber or ink firing chamber 203. The ink firing chamber is further defined by a photodefinable adhesive, or barrier, layer 205 which is applied to a semiconductor substrate 207. The tape 109 is secured to the other surface 208 of the barrier layer (which surface is generally parallel to the surface of the semiconductor substrate) thereby forming one ink ejecting apparatus of the inkjet printhead. Ink, represented by 209, enters the firing chamber 203 via an opening or edge in the substrate 207 thence across the surface 208 of the substrate to the firing chamber 203. Subsequent to the vaporization process which ejects the ink in a thermal inkjet printhead, the chamber is refilled in replacement of the ink which was expelled.
A cross section of the printhead and the cartridge body is illustrated in FIG. 3. It can be seen that heater resistors 201 and 301 may be arranged on opposite sides of the surface of the substrate 207. Conventionally, heater resistors and associated orifices are arranged in staggered fashion (although the heater resistors may also be arranged colinearly) in two essentially parallel rows on the printhead. Ink droplets 303 are selectively ejected upon command of the printer and the responsive electrical heating of the heater resistors. The tape 109, which includes the plurality of individual orifices and therefore is termed an orifice membrane herein, is adhesively secured to a surface of the body member 101 by adhesive 305 in a fashion which places the top surface (as illustrated in FIG. 3) of the barrier layer 205 parallel to and either elevated above or coplanar with a surface 306 of the body member 101. This adhesive further forms a seal around the substrate so that ink, which is in the ink conduit 307, will not leak from around the junction of the orifice membrane tape 109 and the body member 101. An example of a printhead which may benefit from the present invention is further described in U.S. Pat. No. 5,291,226, assigned to the assignee of the present invention.
It has been observed that ink drops ejected from printheads such as those employing tape orifice membranes may be subject to trajectory errors. U.S. Pat. No. 5,467,115, assigned to the assignee of the present invention, suggested a solution to the problem of such trajectory errors. The orifice membrane, there, is affixed to the barrier layer at the surface of the semiconductor substrate using heat and pressure. The orifice membrane extends beyond the outer edges of the barrier layer substrate (as depicted in FIG. 3 herein), bridges the space between the substrate and the cartridge body member, and is affixed to the body member. During the heating and pressure step used to affix the orifice membrane to the barrier layer, the orifice membrane undesirably bends over the outer edges of the barrier layer, causing the orifices to be uncontrollably deformed or tilted thus resulting in inaccurate trajectory of the ink droplets and less than an optimum quality of printing. As a solution, it was proposed that the orifices be pre-tilted prior to the affixing of the orifice membrane to the substrate barrier layer or that the barrier layer be trenched to provide a compensating deformation of the orifice membrane.
In some instances, the solutions offered are insufficient and further correction is needed, particularly those instances when the surface of the barrier layer of the semiconductor substrate is elevated above the mounting surface of the body member and the orifice membrane is then affixed to the body member. Since the surface of the body member is not coplanar with the surface of the barrier layer on the substrate, the orifice membrane is canted away from being planar, thereby stressing the area near the orifices with a tilting of the orifices as a result. Further, a sloping orifice membrane will reduce the area available for ink flow to the ink firing chamber thereby reducing the speed at which the firing chamber will refill with ink.